Crystal structure, phase transitions and vibrations of H2O molecules in [Ca(H2O)2](ReO4)2

Abstract

One reversible phase transition at T_{text{c}}^{{text{h}}} = 261.2 K (upon heating) was observed for [Ca(H2O)2](ReO4)2 in the 120–300 K range. Thermal hysteresis of the phase transition temperature TC of ca. 30 K and heat flow anomaly sharpness suggest that the detected phase transition is of a first-order type. The entropy change value (ΔS ≈ 1.6 ± 0.08 J mol−1 K−1), associated with the observed phase transition (PT), indicates a moderate degree of molecular dynamical disorder. X-ray single crystal and neutron powder diffraction measurements indicated that the crystal space group (I 2/a) is the same for the high- and low-temperature phases. However, FT-IR and RS spectra show a narrowing (during sample cooling) of the bands connected with vibrations of the H2O molecules and ReO4− ions. This suggests that the anions and the ligands from the complex cation perform rapid (picosecond correlation time scale, which is characteristic of optical spectroscopy) stochastic reorientational motions in the low- and high-temperature phases. Moreover, apart from the narrowing, the splitting of some bands can be seen below the phase transition.